In the early part of the past decade, measurements of the ratio of Lambda_b baryon to Bd meson lifetimes, gave results considerably lower than the expectation predicted by HQE theory. This caused concern over the applicability of HQE and various attempts were made to explain the observations. Recent measurements have indicated higher values but with relatively large uncertainties. In this work we present a precise measurement of the ratio and it lends substantial supports to the original HQE theory.